Heat exchangers are devices intended for transferring heat from a first hot fluid to a second fluid which is initially at a lower temperature.
A specific case of heat exchangers is those exchangers intended for cooling the hot exhaust gas for EGR (Exhaust Gas Recirculation) systems through a coolant liquid. These types of heat exchangers must overcome specific technical problems due to temperature changes in their different components.
The temperature variation ranges go from its resting state, where all the components are at room temperature, to the operative mode, where the inlet gases may reach more than 600 degrees centigrade, producing significant differences in temperature in different parts of the device.
The structure of this type of exchangers is configured as a heat exchange tube bundle through which the hot gas circulates, and this tube bundle is housed in a shell through which the coolant liquid circulates.
If the coolant liquid enters and exits approximately at points of the shell located near the ends of the tube bundle, then the flows of gas and liquid circulate approximately according to parallel directions, whether co-current or counter-current.
Problems caused by thermal expansion are solved by making used of intermediate manifolds, which receive or deliver the hot gas, which in turn have bellows-type structures that compensate for the differential expansion between the tube bundle, in contact with the hot gas, and the shell, in contact with the coolant liquid.
A different type of heat exchanger is that consisting of evaporators. Evaporators are heat exchangers designed to transfer the heat of a hot gas to a liquid that is not only heated up but also changes phase.
The technical challenges presented with an evaporator are greater than those of a heat exchanger such as the one described at the beginning of this section. The phase change allows differentiation of three steps in connection with the temperature and the state of the liquid changing phase:
i. step of heating the liquid to be evaporated;
ii. step of phase change;
iii. step of overheating.
The first and second steps occur at not very high temperatures, since the phase change temperature establishes a barrier which prevents raising the temperature above the evaporation temperature. In contrast, the overheating step is not limited by the phase change and may raise the temperature up to values close to maximum temperature values for the hot gas.
The inlet temperature conditions of the two fluids, the hot gas that transfers its heat and the liquid intended for changing phase, are not always the same and neither are the inlet flow rates. The variation of these variables makes the interphase between the first and second step, and the interphase between the second and third step, not occur in the same place inside the evaporator, in connection with the path of the liquid intended for changing phase inside the device, rather it can occur in different places within a certain interval of said path.
Additionally, going from liquid to vapor and going from the mixture of liquid and vapor to superheated vapor is not instantaneous, so no precise place may be identified where the division is established between steps, rather such divisions are in a specific segment.
Each of the steps has different heat exchange conditions. The heat transfer coefficients between the surface of the heat exchange tube and the liquid (step i) are very different from those of a two-phase flow, i.e., the flow formed by liquid plus vapor (step ii), and very different from the heat transfer coefficient of the superheated vapor (step iii).
Not only are the heat transfer coefficients different, but the specific volume in the liquid is very low with respect to the specific volume in the liquid-plus-vapor mixture, and this in turn is low with respect to the specific volume of vapor when the temperature thereof is rising.
All these very different factors between the three steps make the design variables different and the evaporator have technical difficulties that a heat exchanger with no phase change does not show, above all when the evaporator must be compact and occupy the smallest possible space.
Compact heat exchangers are known that are designed to act as evaporators in heat recovery systems in internal combustion engines for impulsion of vehicles. These evaporators increase the heat exchange surface by arranging a tube bundle comprising a bundle of pairs of coaxial tubes. The liquid intended for changing phases passes through the space between the pair of coaxial tubes and the hot gas passes through both inside the inner tube and outside the outer tube.
The fluid changing phase passes between two hot surfaces with little distance between them so that the raising of the temperature and the subsequent phase change takes place within a length of the pair of coaxial tubes that is shorter than if only one tube for circulating the fluid changing phase therein and the hot gas on the outside thereof, were used.
With this configuration one of the problems that exists is that the three heat exchange steps take place throughout the same tubes, so the design of the exchanger cannot be optimized for the three steps at the same time.
As an example of this difficulty, the speed of the inlet flow in liquid phase may be very low due to the low value of the specific volume, while at the outlet, the same liquid flow rate corresponds to a much larger volume of vapor, which imposes much higher speed values than those of the liquid inlet.
Low speed at the inlet can lead to the deposition of dirt and the high speed of the vapor at the outlet can generate excessive pressure drops.
The present invention avoids these problems by using a cross-flow configuration between the hot gas and the fluid changing phase.
The evaporator is constituted, among other elements, by two plates spaced from one another which contain chambers. The heat exchange tubes alternately communicate the chambers of both plates.
The hot gas flows between the plates, parallel to both, in a volume closed by side walls. With this configuration, the exchange tubes are transverse to the flow. The length needed to obtain the vapor at a specific temperature is attained by incorporating the number of tubes needed to reach the length which allows for sufficient heat transfer and therefore cover the three steps.
Expansion of a heat exchange tube depends on the thermal expansion coefficient of the material and on the total length of the tube. With the configuration of the device according to the invention, each of the individual tubes extending between both plates is much shorter than the total length of the path, so the effect of expansion is noticeably reduced.
Another advantage that this configuration has is the possibility of communicating two chambers with more than one heat exchange tube in such a way that, after a phase change takes place, the chambers between which the fluid is being transferred can be communicated with a growing number of tubes. The growing number of tubes is equivalent to an increase in the passage section, and the device thereby takes into account the increase in the specific volume with the phase change, thereby succeeding to lower the speed and thus also the pressure drop.
However, in spite of these advantages, the path that the fluid changing phase follows is more meandering compared to evaporators in which those tubes are parallel to the flow of hot gas, and it has a specific number of intermediate chambers.
The problem set forth by this configuration is the manufacturing thereof, using brazing, since during the passage through the furnace, the gases of the furnace atmosphere, suitable for obtaining good welding, specifically brazing, are not capable of invading the inside of the chambers and the tubes reaching the areas in which the brazing paste is located, above all in those intermediate chambers located in intermediate areas of the path for being spaced from both the inlet and the outlet. Likewise, the oxygen that is inside the evaporator before being introduced in the furnace must be removed, as well as the volatile elements which are formed when the temperature of the brazing paste increases.
The present invention solves this problem by incorporating caps allowing the manufacturing with openings for the easy circulation of gases, i.e., both the entrance of gases from the controlled atmosphere of the brazing furnace and the exit of oxygen and volatile elements, without affecting the advantages this construction provides by means of plates.